A half-wavelength alternating current power transmission system is a novel power transmission system, and its power transmission distance is 3,000 kilometers (50 Hz) or 2,600 kilometers (60 Hz). Compared with conventional alternating current power transmission, half-wavelength power transmission has advantages of no requirement on mounting of reactive power compensation devices, excellent economical type, no requirement on arrangement of an intermediate switch station or the like, which is a potential power transmission manner.
In the conventional power transmission line, distance protection adopts a relatively simpler principle, obtaining calculate impedance capable of correctly reflecting the characteristic of the line after a failure, and can act fast and reliably after the failure, which is widely applied to the power transmission line as a fast protection. However, the half-wavelength power transmission line is extremely long in power transmission distance and the distributed capacitance is large, no reactive power compensation device is mounted on the line, the voltages along the line greatly change. Impedance calculated according to a conventional distance protection principle cannot completely reflect a failure characteristic of the line, and such formed distance protection cannot correctly act on failures in and out of a line region.
Therefore, the conventional distance protection may not be applied to the half-wavelength power transmission line.